I had a theory that the precursors in Halo, that they created the flood.

NOW! I believe this because it has been stated that the precursors are capable of trans-galactic travel, and accelerating evolution of intelligent life. The flood were noted to being completely alien from all biological life in the entire milky way galaxy, meaning they had to have come by some other means.

The forerunners were able to create biological life, so, I have confidence that the precursors were able to create the flood.

The mantle was passed down from precursor to forerunner. The mantle is the duty of keeping biological diversity in the Milky Way Galaxy. If the Precursors intended to release the flood on the galaxy, I can see why they'd want to keep biological diversity, so the flood can mutate a number of different ways.

Precursors discover the flood and bring them into the milky wayWar erupts and the precursors die offForruners build on what the precursors didForunners discover the floodForruners lose thier war and construct the halo arraysthe Halo arrays are fired